Shear-induced smectic ordering as a precursor of crystallization in isotactic polypropylene
Using in-situ small- and wide-angle X-ray scattering, we demonstrate that bundles with smectic ordering are produced by a step shear in a supercooled isotactic polypropylene melt. This occurs well before any formation of crystals. The period of the smectic mesophase is about 3.8 nm and depends neither on the shear rate nor on the temperature. During the subsequent crystallization, crystals grow epitaxially around the smectic filaments that provide nucleation sites. The period of the crystals is an order of magnitude larger than the smectic one. Finally, the smectic filaments also transform into crystals, which is accompanied by a slight increase of the period to 4 nm. A new picture of the shish-kebab structure is presented in which the initial smectic bundles, instead of extended-chain crystals, play the role of the shish, while crystals grow epitaxially around it as kebabs.